This may be off the point, but we had a couple of cats with hyperthyroidism. The older one, who was about 19, there wasn't much we could do for, and he has since moved on. The vet did surgery on the younger one, (who was not that young, she was in her early teens) to remove a portion of the thyroid, and now she's fat and sleek. I don't know how widespread the use of that surgery is, or whether your cat could tolerate it.

Peter.